FIELD OF THE INVENTION
The invention relates to a bipolar silicon transistor having at least one emitter zone with n.sup.+ arsenic doping.
One such silicon transistor is known, for instance, from Einfuhrung in die Halbleitertechnologie (Introduction to Semiconductor Technology) by W. von Munch, Teubner-Verlag, Stuttgart 1993, pp. 208 and 209. That publication describes a bipolar silicon high-frequency transistor with a comb-like arsenic-doped emitter structure. The emitter zones have a very steep doping profile with a low penetration depth (&lt;0.6 .mu.m). Those properties are highly significant for the high-frequency compatibility of the transistor.
The advantage of arsenic as a dopant is based on its low, concentration-dependent diffusion speed, which makes it possible to attain the doping profiles described above. Ion implantation is often employed as a method for producing such doping profiles.
At very high implantation doses, as required, for instance, for an emitter of an HF transistor, crystal damage remains behind in the silicon crystal after an annealing process that can be used for the above-described doping profile. However, crystal damage often leads to impairment of the functional properties of the transistor. Crystal defects located in the region of the emitter, for instance, cause increased leakage currents through the emitter-to-base diode. Moreover, crystal defects in the region of the emitter increases recombination in the emitter volume and causes the current intensity to drop. Such a drop in current intensity has a comparatively marked effect at low currents, resulting in a nonlinear course of the current intensity over the operating current. Moreover, crystal damage can even impair the function of the collector-to-base diode.
The number and electrical effectiveness of crystal defects is generally reduced in semiconductor technology by annealing at adequately high temperatures. For the above-described doping profile, however, no perfect annealing processes exist, since the demand for a large gradient in the drop-off of arsenic dopant concentration in the interior of the silicon crystal and the demand for a low penetration depth of the arsenic dopant does not allow the use of adequately high temperatures and/or adequately long annealing times. The frequency of defects in the known silicon transistors with an emitter n.sup.+ doped with arsenic is therefore very high, which makes for a low production yield.